Fuel Purification Question

[phavoc]

There's been a number of fuel-based questions floating around, and I've always been kinda curious about understanding some things better.

For MGT, the hydrogen fuel is used to power the fusion reactors, but mostly its there to be pumped out into space to create a "pocket universe" that the ship travels in jump space, slowly dissapating until it reaches its destination.

For this you can use either refined or unrefined fuel. Refined fuel being, of course, pure hydrogen stored in a pressurized, liquid form. Unrefined fuel can be collected either from the atmosphere of a gas giant or in liquid form from an ocean (and a few other ways, but lets leave it at that).

So, the fuel purification will filter out the unrefined fuel and store the purified fuel. I'm assuming that any impurities are pumped out into space or ejected in some way. But if you have to filter out impurities (say helium, particulate matter, other gases), wouldn't your total volume of fuel be potentially decreased by the amount you collected? Say you pumped in 100Dtons of water from the ocean into your tanks and you had a fuel purification system that can process 40Dtons/day. That means its going to take you 2.5 days to turn your water into refined hydrogen. But in theory you should lose mass in the purification process. I've yet to find any rule that ever accounted for this in the various versions of Traveller.

So I'm assuming a couple of hand-wavium passes are taking place. First there must be multiple fuel tanks inside your ship to allow you to filter the fuel effeciently. Which would make sense on damage rolls where you only lost a percentage of your fuel since you have multiple tanks (like aircraft do). The other thing is that the fuel purification process is a 1 to 1 ratio of unrefined being converted to refined. From a game playing aspect that makes sense - it's simple, there's no interpretation required for anything.

I guess it would be nice to have little "notes" in the margin, much like the GURPS books that called out these issues to answer the question(s) of this type.

 

[DFW]

Say you pumped in 100Dtons of water from the ocean into your tanks and you had a fuel purification system that can process 40Dtons/day. That means its going to take you 2.5 days to turn your water into refined hydrogen. But in theory you should lose mass in the purification process. I've yet to find any rule that ever accounted for this in the various versions of Traveller.

1 gallon of H2O gets you ~1.5 gallons of H. Similar with liquid ammonia. That's why if you use collapsible fuel tanks in your hold to do a Jn + Jn, you fill the collapsible tanks with water and then run through the fuel refiner and into the main fuel tanks after the 1st jump. That way if you need 20 tons of H for the 2nd jump you only have to carry ~13 tons of water in the hold rather than 20 tons of liquid H...

 

[rust]

Say you pumped in 100Dtons of water from the ocean into your tanks ...

As I understand it, you pump that water into your fuel processor, which
splits it into hydrogen, oxygen and the various impurities, and then pumps
the hydrogen into the fuel tanks while discarding most of the oxygen and
all of the impurities.

 

[phavoc]

Say you pumped in 100Dtons of water from the ocean into your tanks and you had a fuel purification system that can process 40Dtons/day. That means its going to take you 2.5 days to turn your water into refined hydrogen. But in theory you should lose mass in the purification process. I've yet to find any rule that ever accounted for this in the various versions of Traveller.

1 gallon of H2O gets you ~1.5 gallons of H. Similar with liquid ammonia. That's why if you use collapsible fuel tanks in your hold to do a Jn + Jn, you fill the collapsible tanks with water and then run through the fuel refiner and into the main fuel tanks after the 1st jump. That way if you need 20 tons of H for the 2nd jump you only have to carry ~13 tons of water in the hold rather than 20 tons of liquid H...

How do you end up with a larger volume of hydrogen than what you started with?

 

[far-trader]

...

How do you end up with a larger volume of hydrogen than what you started with?

I've not quite grokked that either, especially when comparing room temperature liquid water to cryogenic pressurized liquid hydrogen. Please do enlighten us.

 

[Blix]

http://www.physlink.com/education/askexperts/ae367.cfm might explain it (doesn't appear to agree with the numbers that DFW provides though).

I think it would be better to talk about the mass of hydrogen that you would get, rather than the volume since that is clearly variable depending on pressure and temperature. The mass of one mole of hydrogen is about 1 g ( see http://wiki.answers.com/Q/Find_the_mass_of_1_mole_of_hydrogen_atoms ) so if we get about 210 mols of hydrogen from a gallon of water, that should mass about 0.21 kg.

 

[hdan]

How do you end up with a larger volume of hydrogen than what you started with?

Liquid hydrogen is much less dense than water, so the same mass should take up more volume.

I think.

 

[BP]

Yep - liquid hydrogen is not very dense.

One of the properties of water is its tight molecular structure, which allows it to contain more mass of hydrogen for a given volume, than liquid hydrogen. This is also the case with many other hydrogen-containing liquids...

A cubic meter of water contains ~111 kg of hydrogen, while a cubic meter of liquid hydrogen contains only about 71 kg of hydrogen.

So, like DFW mentioned - a given volume of water would yield a higher volume of liquid hydrogen (actually > 1.56 times by my mathfu).

 

[rinku]

http://www.physlink.com/education/askexperts/ae367.cfm might explain it (doesn't appear to agree with the numbers that DFW provides though).

I think it would be better to talk about the mass of hydrogen that you would get, rather than the volume since that is clearly variable depending on pressure and temperature. The mass of one mole of hydrogen is about 1 g ( see http://wiki.answers.com/Q/Find_the_mass_of_1_mole_of_hydrogen_atoms ) so if we get about 210 mols of hydrogen from a gallon of water, that should mass about 0.21 kg.

Since we know that dTon = metric tonnage in respect to LHyd fuel by Traveller definition, you would therefore need about 4762 gallons of water to produce one ton (1000kg) of fuel. That would be 18,000 litres or so of water, or 18 cubic metres. Which is 1.286 dTons.

Given that water is a lot easier to store and transport, it's still an idea with legs on it. Just allow 1.3 dtons of water for each dton of fuel required.

Edit: I was going on Blix's numbers, but I think he's confused molecular hydrogen and H2. The molar mass of H2 is 2.02g; per the wikianswers it is 210.3 moles of *H2* that you can theoretically extract from a gallon of water, which would weigh 420.6g. So you would only need 2381 gallons of water to produce one tonne of LHYd, or about 9000 litres = 0.64 dtons (which matches BP's figure). In mass terms it's inefficient - if you're running a setting where thrust is based on mass rather than the normal fudge, that 0.64 dtons of water is going to mass about 9 tonnes.

 

[phavoc]

It's been a very long time since I have done any sort of physics, but I'm still a it stumped.

See, If I scooped up 1000 gallons of water, I have a tank filled to the brim of 1000 gallons. According to the math presented, when I stripped out the oxygen molecules from the water, my volume of just hydrogen is now greater than what I had started with.

Now, to me, that seems I'm creating something from nothing. I removed the oxygen, so I would think I would have less volume, not more. I mean, the amount of hydrogen did not increase. All I did was separate it from the oxygen. And since hydrogen is less dense than water, I would think that it would take MORE hydrogen atoms to make up the mass that used to be there when they were connected to the oxygen atom.

Buehler? Buehler?

 

[DFW]

How do you end up with a larger volume of hydrogen than what you started with?

Density. The molecular density is higher with water than L-Hyd.

Liquid hydrogen has a density of 0.07 grams per cubic centimeter, whereas water has a density of 1.0 g/cc. Simple.

Also, although ammonia contains 17.65% of hydrogen by weight, there are 3 hydrogen atoms attached to a single nitrogen atom allows ammonia to contain about 48% more hydrogen by volume than even liquefied hydrogen.

 

[rinku]

The main disadvantage of ammonia over water is that it's toxic, smelly and tricky to handle. I'd take the water option every time.

 

[BP]

P.S. - the other beauty besides storage density and relative safety of using water and ammonia, in addition to their ready availability in solar systems (based on our RW example anyway), is the ability to use the Oxygen and Nitrogen released to help replenish the atmo in the holds...

Note also, that the extra storage capacity of using other liquids is balanced in the game in that it does not reduce required minimal jump fuel requirements (fuel processing takes time) and thus is only good to replenish tanks for jumps and to provide extra operational (m-drive+) fuel - though it could be used creatively there to get some tonnage advantage...

(Not to mention the explosive plot aspects of liberating H and O in a ship travelling 'all alone' in the black :twisted

 

[rinku]

It's been a very long time since I have done any sort of physics, but I'm still a it stumped.

This is actually chemistry .

It's only an increase in volume, not mass. The extracted H2 has less mass than the previous mass of water; as I mentioned above, the water required for one tonne of H2 is going to mass about 9 tonnes. It just fits into 0.64 dTones because of the MUCH higher density of water over liquid hydrogen (as several others have pointed out).

 

[phavoc]

Ok, I get the density issue.

But still, if I have strip out the oxygen from water, I'm still not going to be able to do a 1 for 1 conversion of my water to pure hydrogen. I'm going to need more water to purify if I hope to get the same density of hydrogen as I did for my water right?

So if I go back to my fuel tanks. In one I have 100Dtons of hydrogen, and in the other I have 100Dtons of H20. The amount of hydrogen I have in the pure form will exceed the amount of hydrogen I have in the H20. Or should in my mind at least.

So when you are at starport and fill up with refined (i.e. hydrogen), they would pump in more hydrogen, density wise, than water, right?

I know I'm probably using Dtons wrong here, as we should be talking more in liters, since both are going to be liquids at this point.

And I never liked chemistry...cept the labs. Those were fun.

 

[DFW]

So if I go back to my fuel tanks. In one I have 100Dtons of hydrogen, and in the other I have 100Dtons of H20. The amount of hydrogen I have in the pure form will exceed the amount of hydrogen I have in the H20. Or should in my mind at least.

No, DTons is a measurement of volume. So, 1 Dton (volume) of H2O nets you ~1.5 Dtons of L-Hyd due to the density of H2 in liquid H2O being ~1.5X as much as L-Hyd.

As I 1st stated, it is useful for collapsible tanks where you aren't using it for the 1st jump but are refining it en-route to use for a the 2nd jump of a 2 jump trip.

 

[BP]

...
So if I go back to my fuel tanks. In one I have 100Dtons of hydrogen, and in the other I have 100Dtons of H20. The amount of hydrogen I have in the pure form will exceed the amount of hydrogen I have in the H20. Or should in my mind at least.

So when you are at starport and fill up with refined (i.e. hydrogen), they would pump in more hydrogen, density wise, than water, right?

No - liquid H2 molecules are less closely packed than H2O molecules - there are fewer atoms of H in liquid H2 for a given volume than atoms of H in H2O for the same volume. As you strip away the O from the H2 in water, the separated H2 molecules will expand (and generally the reaction required will release both as gases - before H2 could be chilled back down to liquid temperatures).

Most matter is composed of empty space - the space between molecules and atoms can be tremendous compared to their sizes. And, they are in constant relative motion between each other (till one gets to absolute zero... though they can still be experiencing motion (ex: vibration) at that point, but stationary in relation to one another...).

Molecules 'maintain' (mostly) certain separation distances that determine their macroscopic density. In the specific case of water, as it freezes (and their are several 'freezing' points for water) - its volume changes and thus its density as its mass is retained - hence, 'normal' ice generally floats (i.e in a glass of ice water).

(Please Note: I am being very 'general' in these statements, as reality is a lot more complex (and my teaching skills way too meager) to really adequately cover details and exceptional cases in this medium...)

 

[zero]

Meh, just handwave it and have whatever is purified is what was put in, the handwave being the reasons given above. :roll:

 

[DFW]

Meh, just handwave it and have whatever is purified is what was put in, the handwave being the reasons given above. :roll:

What's put in is purified. As above.